Transducers convert signals from one domain to another and are often used as sensors. For example, acoustic transducers convert between acoustic signals and electrical signals. A microphone is one type of acoustic transducer that converts sound waves, i.e., acoustic signals, into electrical signals, and a speaker is one type of acoustic transducer that converts electrical signals into sound waves.
Microelectromechanical system (MEMS) based sensors include a family of transducers produced using micromachining techniques. Some MEMS, such as a MEMS microphone, gather information from the environment by measuring the change of physical state in the transducer and transferring the signal to be processed by the electronics which are connected to the MEMS sensor. Some MEMS, such as a MEMS microspeaker, convert electrical signals into a change in the physical state in the transducer. MEMS devices may be manufactured using micromachining fabrication techniques similar to those used for integrated circuits.
As an example, a capacitive MEMS microphone includes a backplate electrode and a membrane arranged in parallel with the backplate electrode. The backplate electrode and the membrane form a parallel plate capacitor. The backplate electrode and the membrane are supported by a support structure arranged on a substrate.
The capacitive MEMS microphone is able to transduce sound pressure waves, for example speech, at the membrane arranged in parallel with the backplate electrode. The backplate electrode is perforated such that sound pressure waves pass through the backplate while causing the membrane to vibrate due to a pressure difference formed across the membrane. Hence, the air gap between the membrane and the backplate electrode varies with vibrations of the membrane. The variation of the position of the membrane in relation to the backplate electrode causes variation in the capacitance between the membrane and the backplate electrode. This variation in the capacitance is transformed into an output signal responsive to the movement of the membrane and forms a transduced signal.
Using a similar structure, a voltage signal may be applied between the membrane and the backplate in order to cause the membrane to vibrate and generate sound pressure waves. Thus, a capacitive plate MEMS structure may operate as a microspeaker.
For capacitive MEMS sensors, it is possible, in the presence of moderate physical signals or shock, for one of the deflectable plates to deflect until contacting an adjacent plate. In such cases, the voltage applied to the plates may cause the plates to remain in contact with one another. This phenomenon may be referred to as “pull-in”. In capacitive MEMS sensors, pull-in may affect the performance of the sensor and may lead to reduced performance or failure of the device.